Flexible line trimmers that utilize a spinning trimmer head to flail a short piece of flexible line next to the ground to cut vegetation are well known in the prior art. Flexible line trimmers assume many configurations, depending in part on whether they are powered by an electric motor or a two stroke internal combustion engine. Electric trimmers usually have an electric motor mounted next to the trimmer head on one end of a rigid tube, and at the other end of the shaft, a rear handle for manually gripping. Midway on the shaft is a forward handle. Internal combustion engines, because they tend to be larger and heavier than electric motors, are usually mounted at the end of a rigid tube opposite the spinning trimmer head. This provides better balance and thus comfort of use and control. A flexible drive shaft inside the tube couples the engine output to the trimmer head. Either a rear handle is mounted to the front or rear of an engine housing, or simple grip circumscribing the tube in front of the engine is provided for holding the trimmer in the rear. In some cases, heavy duty trimmers with comparatively large engines are carried on a harness worn by the operator.
The engine, shaft and spinning head create a significant amount of vibration during operation. The vibrational forces are transmitted to the handles of the trimmer and then to the hands of its operator, causing discomfort and possible injury to the operator.
Attempts to reduce transmission of vibration to the operator have been made. Generally, these attempts focus on isolating the front hand grip since the drive shaft carries a great deal of vibrational energy from the trimmer head, the engine and the drive shaft. Although there are many different designs, resilient bushings are typically inserted between the hand grip and the handle or between the handle and the tube to isolate the hand and dampen transmission vibration. Where the hand grip is provided directly on the drive shaft, it is made of a resilient material or has a resilient structure for dampening. Open loop handle configurations have also been used which render the overall handle somewhat flexible.
Vibration isolating and dampening structures do not appear to be generally used to isolate rear handles that are mounted to housings of engines. Engine housings inherently provide some isolation and dampening. So long as the handle is not attached to the drive shaft, there is some dampening. Some other types of portable lawn and garden equipment have used resilient couplings for the rear handle. However, unlike most other types of lawn and garden equipment, a rear handle on a trimmer is required not only to support the weight of the trimmer, but also to support torquing forces applied to the handle by foreaft tilting of the trimmer and twisting of the trimmer, especially where the trimmer is not well balanced.